1. Field of the Invention
The present invention relates to electrical devices, for example circuit protection devices, and to their methods of manufacture. In particular, the devices comprise material that has a positive temperature coefficient of resistance (PTC) and that undergoes a significant and sharp increase in resistance at a specified temperature or over a specified narrow temperature range above ambient temperature.
2. Introduction to the Invention
PTC materials, which may be polymeric or ceramic, are known for use in electrical devices such as heaters, and also for protecting electrical circuits against excessive current or temperature. The excessive temperature may itself arise simply from current flowing through the device, or may be due to an increase in the ambient temperature beyond a desired value. Details of developments relating to conductive polymer PTC compositions and devices comprising them, are given for example in U.S. Pat. Nos. 4,017,715, 4,177,376, 4,246,468, 4,237,441, 4,238,812, 4,329,726, 4,255,698, 4,272,471, 4,445,026, and 4,327,351, and GB 2,038,549. It has been proposed to use devices comprising PTC elements to protect circuits against fault conditions arising from excessive temperatures and/or circuit currents in for example U.S. Pat. Nos. 2,978,665, 3,243,753 and 3,351,882, U.K. Pat. No. 1,534,715, the article entitled "Investigations of Current Interruption by Metal-filled Epoxy Resin" by Littlewood and Briggs in J. Phys D: Appl. Phys, Vol. II, pages 1457-1462, and the article entitled "The PTC Resistor" by R. F. Blaha in Proceedings of the Electronic Components Conference, 1971, and the report entitled " Solid State Bistable Power Switch Study" by H. Shulman and John Bartko (August 1968) under Contract NAS-12-647, published by the National Aeronautics and Space Administration. U.S. Pat. Nos. 4,238,812 and 4,255,698 disclose practical circuit protection devices comprising conductive polymer PTC elements.
The disclosure of each of the patents and publications referred to above is incorporated herein by this reference.
The present invention is concerned particularly, though not exclusively, with electrical devices comprising PTC material, preferably polymeric, for use in circuit protection, and U.S. Pat. Nos. 4,238,812 and 4,329,726 referred to above for example, disclose such devices. PTC circuit protection devices are such that under normal operating conditions, determined by the current and temperature rating of the PTC material, they exhibit very low resistance to the flow of current therethrough. Under fault conditions, of excessive current and/or temperature, the PTC material heats up, significantly increases its resistance and thus switches off the flow of current therethrough, thereby protecting an associated electrical circuit. The higher the resistance of the material after passing through the switching transition, the lower is the residual current that can flow through the device, and thus the more effective is the device in protecting its circuit. Products embodying these principles are presently sold by Raychem Corporation under its tradename POLYSWITCH. Some of such products are formed from a substantially homogeneous sheet of polymeric PTC material. The sheet is coated over each of its major surfaces with electrically conductive material, to act as electrodes, and disc, rectangular, or other, shaped devices are stamped therefrom.
The resistivity of the PTC material of POLYSWITCH devices typically does not exceed 10 ohm-cm, and taking a typical value of 5 ohm-cm and a disc configuration with typical dimensions of diameter 2 cm and thickness 0.05 cm, the resistance at room temperature (i.e. about 20.degree. C.) of the device is typically 0.08 ohm. Although other geometries, for example rectangular, can be employed, the resistance values provided by devices that are of a size that can conveniently be handled manually do not vary significantly from the values given above. If it is desired to make such devices of different resistance, then this can be arranged in various ways: (a) a different PTC material having a different resistivity can be employed. However, it has been found that polymeric materials in sheet form suitable for circuit protection devices and having a resistivity greater than about 10 ohm-cm cannot reproducibly be manufactured in significant volume. This makes it very difficult to produce devices having a significantly higher resistance. This is, of course, not necessarily true for polymeric PTC materials in general, but has been found to be so for materials needed for circuit protection devices in view of their need to have a relatively sharp transition from low resistance to high resistance as the temperature increases, for example due to a current increase above a threshold value; (b) the separation of the electrodes can be varied, but for sheet material it is difficult reproducibly to increase the thickness enough to produce a suitable device having a significantly higher resistance. The difficulty encountered is that of providing, by extrusion for example, a relatively thick sheet of a polymeric material that is highly loaded, for example by as much as 50% of its volume, with fillers such as carbon black, in which the composition of the material is homogeneous throughout. In this respect, it should be noted that a typical width of extruded sheet is 30 cm; and (c) the diameter (or other planar) dimension of the stamped product can be varied. However, below a certain size it becomes very difficult physically to handle the individual devices, so again there is a problem with producing devices of higher resistance. For these practical reasons therefore it is difficult to make such devices having a resistance greater than say about 5 ohms. However, there is a requirement for higher resistance devices in order to limit current in electrical or electronic circuits to less than about 200 mA at normal operating voltages. In this respect, it is to be noted that the power generated in the device (I.sup.2 R) must be large enough to raise the temperature of the PTC material up to the temperature at which its resistance increases sharply, so that the lower the current flowing through the device the higher must be its resistance.
Other POLYSWITCH products produced by Raychem are of strip rather than disc form, but with strip configuration, the short length of the strip needed for a sufficiently high resistance device to protect circuits adequately against currents below about 200 mA would be too small for easy handling.
EP-A-0 087 884 discloses a further polymeric PTC circuit protection device in which a cylindrical element of PTC material is mounted within an enclosure between cup-shaped electrodes at each end thereof.